For data or command transmission or reception, for example within a short range, a conventional transmitter or receiver uses frequency shift keying. If the RF carrier frequency is high, for example on the order of 2.4 GHz, a relatively high bandwidth is selected for the intermediate frequency, in particular, higher than or equal to 200 kHz. The modulation frequency deviation in the modulated signals can be adapted according to the bandwidth. In such case, a frequency reference may be used, provided by a local oscillator, which might not be very precise and is thus inexpensive. However, account must be taken of the power of thermal noise, which is proportional to the selected bandwidth. Thus, a broadband transmission or reception system generally does not have excellent sensitivity.
When a local oscillator with an inexpensive quartz is used, the frequency of the oscillating signals provided by the oscillator may vary by around ±20 ppm. For frequency conversion via a mixer unit with incoming radio frequency signals at 2.4 GHz, the frequency error in the oscillating signals produced by the local oscillator may thus be around ±100 kHz. In order to increase the sensitivity of the receiver, it may be preferable to use radio frequency signals with a low data rate, for example on the order of 1 kbit per second. However, even if the intermediate signals provided by the mixer unit are immediately subjected to lowpass or bandpass filtering, this does not allow precise filtering to be subsequently performed to obtain a high sensitivity receiver. With precise filtering by means of narrow band filters, the intermediate signal frequency is likely to be outside the frequency bands of the narrow band bandpass filters. In these conditions, the data or commands in the incoming radio frequency signals cannot all be extracted after the demodulation stage, which is a drawback. It is thus generally difficult to use an inexpensive quartz, if it is hoped to perform data demodulation after narrow band bandpass filtering. The frequency error in the intermediate signals would definitely have to be corrected.
In the case of a high data rate PSK (phase shift keying) radio frequency signal receiver, a synchronous approach to data demodulation is possible. This differs from the receiver of the present invention, which uses an asynchronous, and thus uncorrelated approach.
WO Patent Application No. 96/32797 A1 discloses an MFSK radio frequency signal receiver. This receiver takes account of the effects linked to the movement of the receiver and obstacles on the path of the transmitted radio frequency signals. The receiver can receive a signal at a carrier frequency within a determined frequency range. The incoming signals are amplified and filtered prior to frequency conversion in mixers by in-phase and quadrature oscillating signals from a local oscillator. Intermediate in-phase and quadrature signals at the outputs of the mixers are filtered by lowpass filters prior to being sampled, each in respective samplers. A set of N samples is stored in a buffer memory. A discrete Fourier transform (DFT) is performed on the N samples stored and the result of the discrete Fourier transform is stored in a buffer output memory. The local oscillator frequency is not adjusted, which prevents the intermediate signal frequency from being centred. Consequently, high sensitivity data demodulation cannot be performed, which is a drawback.
US Patent Application No. 2003/0203729 A1 mainly discloses frequency compensation in a GFSK radio frequency signal receiver. The frequency of the incoming radio frequency signals may be between 2.4 and 2.4835 GHz, on the order of 2.4 GHz on the Bluetooth network. The local oscillator frequency must be adjusted to remove any frequency deviation relative to the frequency of the incoming signals. To achieve this, an incoming signal peak detector is provided to determine a maximum positive peak value and a maximum negative peak value for the incoming RF signals. The middle point between the positive and negative peak values represents the detected central frequency. This thus allows the oscillator frequency to be adjusted to the desired frequency in the automatic frequency compensation (AFC) loop for data demodulation. Determination of the frequency deviation or shift is performed at a high frequency to enable adjustment of the oscillator frequency, and there is no provision for reducing the electric power consumption of the receiver, which is a drawback.